Signal blocking and fading in mobile communications systems is well known. Satellite-based systems generally have more stringent requirements that terrestrial-based systems due to the significantly longer propagation paths. In satellite mobile communications systems blocking and fading of user terminals by buildings, trees and terrain can be mitigated by using multiple orbiting satellite repeater transmitters to send multiple copies of a signal, via some or all satellite repeater transmitters in view, to a user which is potentially experiencing signal blocking and fading. These mitigation techniques, especially those using spread spectrum systems, utilize multiple signal path diversity (hereinafter referred to simply as "path diversity") as a means of maintaining communication paths when individual mobile users are in blocking and fading situations. Low Earth Orbit (LEO) satellite communication systems in particular can exploit path diversity since there are multiple satellites and, hence, multiple and different communication paths to and from the user.
Most known or proposed systems of this type, in addition to using Code Division Multiple Access (CDMA), generally channelize by frequency division multiplex (FDM). Furthermore, providing path diversity has an adverse affect of requiring the system to utilize many satellites. This increases the total power demand for each satellite, and also requires each satellite to make the same RF channels available for each user for path diversity transmissions. The end result can be a reduction in the total capacity of the system due to inefficiencies in RF channel assignment.
One approach to providing path diversity is to provide path diversity indiscriminately to all users. In fact, however, the inventor has realized that there are many different types of user terminals, as well as many different types of communication environments that a given user may reside in, either temporarily or permanently. For example, certain users will employ vehicle mounted terminals which can move through the environment rather quickly. Other users may employ hand-held or fixed terminals which may not be moving at all. In addition, there are a variety of terrains wherein users can be located, such as oceans, deserts, forests, suburban, urban, rural farmland, etc.
It can be appreciated that not all communication environments require the same level of path diversity, and furthermore not all user terminals within a given environment require the same level of path diversity.